1. Field of the Invention
The present invention is generally related to gas compressors and pumps. More particularly, the present invention is related to positive displacement rotary compressors, specifically including those known as Roots blowers and compressors.
2. Description of Related Art Including Information Disclosed Under 37CFR 1.97-1.99
The present invention is related to, and constitutes an improvement over, the rotary gas compressors disclosed in the applicant's previously issued U.S. Pat. Nos. 4,859,158, 5,090,879, and 5,439,358, issued Aug. 22, 1989, Feb. 25, 1992, and Aug. 8, 1995, respectively.
The class of positive displacement compressors known as Roots blowers has been known to and has served industry continuously since the mid 1850's. For certain applications, the Roots blower offers a number of advantages over other types of gas compressors, including conventional reciprocating piston compressors, helical screw compressors, fan-type blowers, centrifugal and roto-dynamic compressors. Among the advantages of the Roots blower are simplicity, ruggedness, trouble-free operation, and high volumetric capacity. Roots blowers do not contaminate the gas being processed, as there are no valves or reciprocating, rubbing, or contacting mechanical parts in the flow stream. The Roots blower maintains constant volume displacement from intake through to discharge, a design feature not found in any other type of positive displacement compressor.
Roots blowers incorporate two lobed impellers, sometimes called rotors, which mesh with one another and which are driven in opposing directions through timing gears attached to each drive shaft. Commercially available Roots blowers usually have impellers with either two or three lobes. Roots blowers have also been designed to incorporate impellers having four or more lobes. Two-lobed impellers have the greatest volumetric capacity per revolution, and are the most common. Volumetric capacity is reduced proportionately by adding additional lobes. The Roots blower excels in moving large volumes of air or other gases against low pressure differentials. Typical applications include compression from atmospheric pressure to from 5 to 7 psig discharge pressure, and non-contaminating evacuation, serving either as a vacuum pump or as a vacuum booster.
Roots blowers have not heretofore been useful for or capable of compressing a gas against a substantial pressure differential. This limitation has been due to heating effects that attend such compression. As a gas is impelled through a conventional Roots blower it is compressed and heated as it enters the discharge region. Such compression is adiabatic, such that the temperature of the gas increases exponentially with increasing pressure ratios. Additional heat resulting from dynamic flow effects is generated as discharge pressure gas surges into impeller cavities and is then expelled in the opposite direction.
The increase in the temperature of the gas leads to heating of the impellers, the housing, and other mechanical parts of the blower. This in turn can lead to thermal distortion, expansion and contact between interior components. At pressure ratios of about two to one (2:1) such effects become a significant problem and essentially limit the sustained operation of the blower. Overheating of the blower can result in lockup or other mechanical failure of the impellers, seals, and other components. This heating problem is not uniform throughout the compressor. The compressor housing, for example, can be externally cooled by a number of conventional methods such as the use of water jackets, heat radiating fins, heat sinks, and the like. The greatest heating problem lies with the impellers, because there is no practical way to directly cool them. Overheating of the impellers leads to their expansion and eventual binding against the housing, causing extensive damage and shutdown. Overheating has been a major limitation on the use of Roots blowers for compressing gas against high pressure differentials.
A significant advance in the art was the development of recirculation cycles to effect a moderate reduction in the heating of Roots compressors. In a recirculating Roots compressor, a portion of the discharge gas, which is compressed to a higher pressure than the input gas, is recirculated back into the compressor so as to effectively increase the pressure of the gas passing through the compressor. In some recirculating compressors a portion of the discharge gas is cooled prior to being recirculated back into the compressor. In both cases the operating temperature of the compressor is effectively reduced, thereby mitigating the overheating problem referred to above. By this means, a capability for sustained operation has been obtained in some cases up to pressure differentials of approximately 2.7:1.
U.S. Pat. No. 2,489,887 to Houghton, for example, discloses the general concept of cooling a Roots compressor by introducing recirculated gas of a lower temperature into the intake gas to reduce heating of the compressor.
U.S. Pat. No. 3,351,227 to Weatherston discloses a multi-lobed Roots-type compressor having feedback passages which allow a portion of the high-pressure discharge gas to be recirculated back into the pump housing. Weatherston however discloses only the use of quite small feedback passages, the size of which are not related to the sizes of the intake and discharge ducts. This results in uneven flow velocities and pressures. As will be apparent from the description of the present invention set forth below, the Weatherston compressor does not solve problems addressed by the present invention.
German Patent No. 2,027,272 to Kruger discloses the concept of cooling and recirculating discharge gas in a two-lobe Roots compressor. The compressor of Kruger, due to its two-lobed configuration, has no provision for preventing communication and backflow from the discharge port into the recirculation ports.
French Patent No. 778,361 to Bucher discloses four-lobed Roots compressors having recirculation ports. The recirculation ports are however small, with the intended purpose of using small nozzle-like ports to allow the recirculated gas to adiabatically cool upon entry into the compressor housing. As will be made apparent from the description below, this teaching of Bucher is contrary to the present invention.
U.S. Pat. No. 4,453,901 to Zimmerly discloses a positive displacement rotary pump, which is designed for pumping liquids, with no provision for recirculation.
U.S. Pat. No. 4,390,331 to Nachtrieb discloses a rotary compressor having four-lobed impellers, but likewise having no provision for recirculation.
U.S. Pat. No. 2,906,448 to Lorenz discloses a rotary positive displacement compressor having two-lobed impellers, with a double-walled construction for cooling purposes.
British Patent No. 282,752 to Kozousek discloses a rotary pump which is characterized by rotor lobes that are particularly shaped so as to provide the maximum possible working space and thereby maximize the volumetric capacity of the pump. The pump disclosed in Kozousek discloses recirculation ports which are made small, and which are for the purpose of obtaining even delivery of the gas.
Various kinds of Roots compressors are commercially available, both with and without recirculation. However, none of the commercially available compressors address the problems of recirculation flow impedance and recirculation port flow dynamics, which are addressed by the present invention.
In some prior art recirculating Roots compressors, such as the compressor described in Houghton, the flow of recirculating gas is periodically interrupted each time a rotor lobe passes the recirculation entry port, or is halted and possibly even reversed as a displacement cavity is simultaneously opened to both a recirculation port and a discharge port. This results in a loss of momentum and flow of the recirculation fluid, creating heat, and reducing the efficiency of the recirculation fluid in cooling the compressor flow. This problem, which is inherent in many previously known Roots compressors, is overcome in the present invention, as will be made apparent in the descriptions set forth below.
In the applicant's previously issued U.S. patents cited above, certain improvements were disclosed which achieved lower operating temperatures by recirculation of the working fluid which usually required cooling for most applications. The present invention provides certain improvements in the compressors described in those patents such that the thermodynamic nature of the compression cycle has become significantly more isothermal than adiabatic, such that substantially less heat is generated in the process.
Accordingly it is the object and purpose of the present invention to provide an improved positive displacement, transverse flow, rotary gas compressor.
It is also an object and purpose of the present invention to provide a positive displacement, transverse flow rotary gas compressor having an improved gas recirculation means for reducing overheating of the compressor.
It is a further object and purpose of the present invention to provide a positive displacement rotary gas compressor which is characterized by having a continuous, steady uninterrupted flow of recirculation gas which flows from the discharge of the compressor back into the compressor.
It is also an object and purpose of the present invention to provide a rotary, positive displacement, transverse flow gas compressor that produces significantly less heat inside the compressor, and is thus capable of operating at higher sustained pressure ratios than have previously been attainable.
It is also an object of the present invention to provide a positive displacement, transverse flow, rotary gas compressor which establishes a compression cycle having a thermodynamic nature that is significantly closer to isothermal than to adiabatic, and which does not require internal cooling for operation at pressure ratios of up to ten to one (10:1).
It is yet another object of the present invention to provide a positive displacement, transverse flow rotary gas compressor which achieves improved efficiency through a substantially isothermal thermodynamic compression cycle.